By multiplexing two or more plasmon-resonance perfect-absorber structures together, wideband performance in the infrared is achieved.
JOSHUA HENDRICKSON and JUNPENG GUO
Anomalous light absorption in metal structures was first observed by R.W. Wood a century ago.1 Interest in strong light absorption in metallic structures resurfaced in the 1960s.2-9 Metamaterial-based perfect light absorbers are metal plasmonic-resonance structures that completely absorb incident light at specifically designed wavelengths.
Because light absorption in structured metals is due to surface-plasmon resonance, perfect absorption typically occurs at a specific wavelength with a very narrow spectral range. In many applications, however, it is desirable to have perfect light absorption over a broad spectral band.
Multiplexed structures
Recently, a wideband perfect light absorber in the midwave IR was proposed and demonstrated by using multiplexed metal structures.10, 11 In the multiplexed plasmon-resonance structure, several gold metal squares of different sizes are multiplexed in the unit cell. The multiplexed-structure perfect absorber can completely absorb photons falling onto the surface over a certain spectral band due to the multiple resonance modes and the coupling between these resonance modes.
The regular-structure (narrowband) perfect light absorber can be seen in Fig. 1a. Gold thin-film squares are periodically patterned on the top of a thin dielectric layer deposited on top of a thick gold metal layer. The gold layer is thick enough so that no transmission can occur. Due to the plasmonic resonance in the structure, optical reflection from the surface can be eliminated.
The multiplexed metal-structure perfect light absorber is shown in Fig. 1b. The period of the multiplexed structure is the same as the period of the nonmultiplexed structure; but in the multiplexed structure, there are two different-sized metal squares in the unit cell that generate two plasmon-resonance modes at different frequencies. In both of these structures, the periods of the unit cells are identical in both lateral dimensions to ensure polarization-independent absorption for normal incidence.
Wideband performance
In one example, a multiplexed perfect-absorber structure has a unit cell in which two gold film squares of 815 nm and 865 nm sizes are multiplexed (see Fig. 2). Optical power reflectivities from perfect absorbers with a multiplexed metal structure and with a nonmultiplexed structure are shown in Fig. 3. The dotted blue line is the reflectivity from the nonmultiplexed regular structure perfect absorber with an 815 nm gold square in the unit cell. The device has near-perfect absorption of 96% at a 3.36 µm wavelength. The dotted black line shows the optical reflection from the regular nonmultiplexed-structure perfect absorber with an 865 nm gold square in the unit cell. This device has near-perfect absorption of 96.7% at a 3.55 µm wavelength.
The solid red line in Fig. 3 shows the optical power reflectivity from the multiplexed structure perfect absorber with both 815 nm and 865 nm gold metal squares in the unit cell. The multiplexed-structure absorber reaches above 97% over a wide spectral band centered at a 3.45 µm wavelength. The multiplexed structure’s absorption band has been expanded significantly due to the two gold metal squares of different sizes in the unit cell.
This absorption-band expansion is not a simple linear superposition of two absorption bands of the regular nonmultiplexed metal-structure perfect absorbers. The coupling of two resonance modes may also contribute to broadening of the absorption spectral band.
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Joshua Hendrickson is a research physicist at the Wright Patterson-Air Force Research Lab (Dayton, OH) and Junpeng Guo is a faculty member of optics at the University of Alabama (Huntsville, AL); e-mail: [email protected].